1. Field of Invention
The present invention relates to a purification method for obtaining high-purity pyromellitic dianhydride (PMDA). More particularly, the present invention relates to using a temperature-controllable cyclone separator to obtain high-purity PMDA.
2. Description of Related Art
Polyimide (Pl) is a kind of high-performance engineering polymer. Polyimide can be used as an insulating film for components of electro-machines and microelectro-machines. It also can be used as a soft substrate for semiconductors. PMDA is the monomer of Pl thus the required purity of PMDA is very high.
Traditional purification processes for PMDA are extracted by solvent or purified by crystallization. Furthermore, the PMDA obtained is a very fine powder, therefore it is not convenient to use PMDA. Moreover, the purification process is complicated and produces effluent that needs to be handled. The waste effluent treatment causes a lot of problems.
For improving these disadvantages described above, the more recent developed method is to use desublimation method to refine the powdered PMDA product. Most methods disclosed in patents utilize the difference of dew point between PMDA and other components to separate the PMDA. For example, in JP 8-59668, a shell and tube heat exchanger was applied to purify PMDA. PMDA vapor contained stream, which is produced from a PMDA synthesis reactor or by heated crude PMDA, is passed through the tube of heat exchanger while a coolant is pass through the shell side counter currently. The temperature of outlet gases is cooled by coolant, and the resultant solids and gases are separated because sublimation temperatures of each component of outlet gases are different. However, if the outlet temperature is deviated from the set temperature slightly, a lot of powder will nucleate on the tube wall of the separator and block the tube.
Another method is to cool the outlet gases by quenching air to obtain PMDA powder such as described in JP 61-215352. Although bigger crystal of PMDA can be obtained by this method, impurities are often encompassed in PMDA crystals. Therefore, high purity of PMDA is not easy to obtain.
Using a bag filter to collect PMDA powder and performing succeeding purification steps is another method to purify PMDA, but water is often accompanying in product,therefore the resulting PMDA agglomerates powder always block the filter of the bag. Still another method is to use scrubbers to wash PMDA powder by using water to collect PMDA powder, then PMDA powder is dehydrated and purified.
There are some other pertinent patents such as JP 07188242, JP 0518473, JP 05078367, JP 63010790, JP 62093291, JP 01186839, JP 03294272, JP 59199683, Ger. Offen DE 87-3732-0747, and Ger. Offen DE 3501371, etc. Some of these patents utilize the desublimation method to purify PMDA. Some of these patents utilize organic solvents to extract PMDA, and then recrystallization is used to purify PMDA. The organic solvents used are a mixture of acetone, o-xylene and cyclohexane, or benzonitrile. Some of these patents hydrolyze PMDA to pyromellitic acid (PMA), then perform dehydration reaction to gain purified PMDA.
The invention provides a method for producing high-purity PMDA with larger size crystals.
This invention provides a purification method for obtaining high-purity PMDA. A reactor outlet is connected with an inlet of a first cyclone separator in series. The temperature of the reactor outlet is higher than a sublimation temperature of PMDA. The temperature of the first cyclone separator is lower than the sublimation temperature of PMDA and higher than that of by-products. Vapors of the PMDA and the by-products are directed to flow from the reactor outlet to the inlet of the first cyclone separator at a flow rate to obtain high-purity PMDA.
A temperature of the reactor outlet is, about 245 to about 255xc2x0 C. The first cyclone separator has a temperature of about 165 to about 210xc2x0 C. Furthermore, more than one cyclone separator can also be connected to the first cyclone separator in a series. A temperature of these succeeding cyclone separators is lower than sublimation temperatures of the by-products.
Therefore, this invention provides a very simple purification method for obtaining high-purity PMDA. Only one simple purification step can obtain high-purity PMDA (purity greater than 99.0%). The complicated purification steps are largely simplified in this invention. The cost of producing high-purity PMDA is also largely decreased. Therefore, this invention is very suitable to be applied in mass-production of high-purity PMDA.